Sick Cories

Question
Karen,

I have a 55 gallon, fully cycled tank that I have had for about 7 months.  Temp 78, pH 7.4, ammonia & nitrite 0 and nitrate 20.  I am not overstocked and know a fair amount about freshwater.  I have 4 Schwartzi cory cats in my tank, they have always done well, have had them about 5-6 months or so.  I changed all of my gravel from tan to black today.  I rinsed the gravel thouroughly in old water from the tank.  I did a 30% partial water change to the tank because the nitrates were about 40 prior to the water change, but kept all of the same plants and the same filter (Aqua Clear 40-70).  All of the other fish are fine (platies, mollies, tetras, zebras, hatchets).  My cories initially ate and acted okay when all was finished too late this morning.  A few hours later I noticed all four are now belly up barely breathing on the bottom of the tank and attempting to hide.  What did I do wrong?  Can I save them?

Answer
Hi April,
Aw, poor babies. I'd do a 50 water change right away and add an ammonia dechlorinator if possible.

The complete gravel change could have wiped out the majority of your biofilter causing an ammonia spike. Testing your water will really tell all now. Cories are a lot more susceptible to ammonia.

Water changes should help them now. If you have ammonia dechlorinator make good use of it.

Best wishes and I hope your cories make it alright!
Karen~